Z39

Z39 is short for the Zenit M39 mount – a lens mount using a 39 mm thread (like the Leica Thread Mount), but a flange focal distance measuring 45,2 cm.

Zenit M39 lenses were used on early Soviet Zenit SLR models (Zenit 1, Zenit-C, Zenit 3, Kristall and Zenit-3M).

Adapting Zenit M39 lenses to modern cameras is possible. There are dedicated adapters, but most of them are cottage industry products. Alternatively, you can use a combination of an Z39->M42 and M42->Your system adapter (but infinity might be off my a bit) or an LTM->Your system adapter with a 16,4 mm extension tube (these are relatively easy to come by, as they were even in the Soviet era produced en masse).